(31-12-2010, 11:08 AM)PJW Wrote: (30-12-2010, 10:52 PM)Archie Wrote: Hi, would someone be so kind to tell me, within a LOC on 'new works' why we use the pink 1.0mm wire for points relays, and not the standard 0.75mm white, it is obviously a bigger cross section, is it a case of less resistance, if so why is this significant. There is probably a very simple answer to this I am not sure of, but I was working in Reading over Christmas doing 'changeovers' and been back on the NLL today doing Mods for the testers, and it has come into my brain on the way home as a question to ask.
Many thanks
You are right that the larger wire size is used so that the resistance is lower- not really a matter of current carrying capacity as such but lowest possible voltdrop is important.
The difference in size may not seem a lot but remember that the ratio of the cross sectional area is approx 2:1. It is used for the points drive circuits and thus just for the wiring from the busbar to the outgoing cable terminations via the points contactors. The outer diameter of the two wires are actually very similar, which is why pink insulation has been selected for the larger wire is used, so that the two can be readily distinguished.
There's a relationship here to the normal current, potential fault current and fuse value protecting the circuit.
A points drive circuit is expected, in normal circumstances, to draw higher levels of current (about 5 amps) than a simple relay circuit (about 50 milliamps), so 100 fold greater. The fuse protecting the circuit must be able to withstand this level without nuisance trips - hence points circuits typically having 5 or 10 amp fuse, instead of 2 or 3 amps for a normal relay circuit - the latter is clearly way over rated (but not a problem) compared to normal current levels, but is the smallest "standard" size we typically stock.
There's now a relationship between wire size and the rating of the fuse protecting the circuit. The idea is that if there is a "dead short" between B and N (eg cable cores squashed together when damaged), the wires/ cable cores feeding that fault could then heat up and cause a fire, which would be far more damaging than the original fault. So what you want to happen is that if there's a "dead short", the fuse protecting the circuit should blow. (the fuse wire in the glass envelope heats up and melts, instead of the loc wiring!). the relationship between wire cross section and fuse rating is shown in the attached table, extracted from NR standard 11600.
So this means that, because the fuse rating is higher (to avoid nuisance trips), the wire size must also be thicker (to stop it catching fire), regardless of volts drop issues.
Finally, there's a practical limitation due to how thick a wire will fit in the crimp in use - 1.0 sqmm for an SSI plug coupler crimp, and 1.15 sq mm for a Q style crimp. They can both be a struggle to get all the strands in. The latter is also able to take two wires side by side, so they are used in parrallel to create a "ring main".
Finally finally (!), the standard type C cable core is 2.5 sq mm, so is already quite thick enough already, but can just look like a "wire" on the diagram.
Reuben
